I've had this issue for ages with my HTC One (M7) - don't know how I missed this thread in my searches.
I believe the fact that all audio on the M7 is routed through the Beats Equaliser is what is breaking the gapless playback (as it'll be bypassing the stock Android audio API that allows for gapless playback). That's my theory anyway, as it does seem to work fine on the Google Play Edition version of the M7 (which doesn't have Beats software equaliser). There's no way of activating the Android stock equaliser on the M7 that I can see either.
No way I'm rooting it though and missing out on all the nice HTC rom touches.
I've reported it multiples times within the app but I'm not sure who's problem it is to fix - Google or HTC...
I'm seriously considering ditching sense. Even though I love blink feed and zoe, I'm pretty sure I'll find something third party that does the trick. What I can't find is something that works completely with my google music and I use my phone mostly as a personal music player more than anything. This is a joke , the so called boom box that can't play music gaplessly. HTC or google fix this. Why does Poweramp play gaplessly and google music cannot on my HTC One.